Porcine reproductive and respiratory syndrome (PRRS) is one of the major infectious diseases threatening pig industry in many places around the world. Ever since the outbreak of highly-pathogenic porcine reproductive and respiratory syndrome (also called highly-pathogenic blue ear disease) in China in 2006, PRRS has caused huge economic loss to Chinese pig industry, and is listed by Chinese Ministry of Agriculture as one of the diseases for which compulsory vaccination is required.
In addition to PRRS, pigs can be further infected by other infectious diseases such as classical swine fever (CSF) and pseudorabies. However, PRRS virus (PRRSV) is known to induce immune suppression after infecting its host, and therefore usually result in reduced immune response to secondary infections or even vaccination failure. Studies have shown that PRRSV impairs host immune system by, for example, destroying alveolar macrophages that are important for generating immune response, and/or suppressing cytokine expression that confers immunological defense to secondary infections. For example, PRRSV infection has been found to significantly inhibit host immune response to Classical Swine Fever Virus (CSFV) vaccine, even resulting in CSFV vaccination failure (Suradhat, S. et al, Vaccine, 24: 2634-3642 (2006); Li, H. et al, Veterinary Microbiology, 95: 295-301 (2003)). Co-vaccination of attenuated PRRSV and attenuated CSFV is reported to have a reduced immuno-protection rate of about 60%, which fails to meet the vaccination requirement. To vaccinate against the two pathogens, individual vaccinations separated by a 14-day interval are required (see, e.g. Du, X. Z. et al, Zhejiang Journal Animal Science and Veterinary Medicine, 2: p 5-6 (2011)). For another example, PRRSV has been found to negatively affect the vaccination effects of Pseudorabies Virus (PRV), and significantly reduce or delay the host immune response against PRV (De Bruin, M. G. M. et al, Veterinary Immunology and Immunopathology, 76(1-2): p 125-135 (2000)).
The immuno-inhibition of PRRSV tends to complicate the vaccination regimen for pigs, and reduce the vaccination efficacy and efficiency. When pigs are vaccinated against PRRSV and other viruses, it is often necessary to apply repetitive injections and multiple dosages, making the vaccination process time-consuming, labor-intensive, and costly. Moreover, in a multiple vaccination regimen, a missing dose can have a direct impact on the protection efficacy of the vaccines, while frequent and repeated vaccinations can result in immuno-paralysis, and induce immunological stress.
Therefore, there exists great need for a combined vaccine composition for PRRSV and other pig infectious diseases, without substantial immuno-inhibition.